1. Field of the Invention
The present invention relates to the formulation of a bath enabling the preparation of a deposit of nickel-phosphorus (Ni-P) alloy on substrates used in the manufacture of printed circuit wafers and more particularly, to deposition of such alloys by chemical self-catalysis.
2. Discussion of the Related Art
Existing nickel baths are all relatively short-lived. Typical nickel baths include combinations of complexing agents, buffers and stabilizers as well as nickel salt and phosphites. Typical complexing agents have included lactic acid, maleic acid, aspartic acid, propionic acid, succinic acid, citric acid, etc. Several different complexing agents are generally associated with one another. It is known that the complexing agents, and especially citric acid or its derivatives, are incapable of tolerating large quantities of phosphites. The phosphite content determines the quantity of phosphorus that can be deposited, and hence the lifetime of the bath. Therefore, depletion of the bath's phosphite content decreases the bath lifetime.
Due to the short lifetimes of baths that presently exist in the market, it is very difficult to create conditions for making the deposits. It is known that a small quantity of citric acid or of one of its derivatives causes a rapid decomposition of these baths by the precipitation of nickel phosphites, therefore, citric acid and its derivatives are not typically used as complexing agents. In addition, stabilizers are reputed to be poisons in the self-catalytic reaction, and therefore require handling with care. At present, the use of gluconates has been limited to cleansing solutions.
Therefore, due to the difficulties associated with creating the conditions necessary for making such deposits, the quality of the coatings is reduced and their cost price remains very high.